1. Field of the Invention
The present invention relates to a toner for use in the electrophotographic copying machine, laser printer, and facsimile apparatus, and more particularly to a yellow toner for use in the electrophotographic color image formation process.
2. Discussion of Background
In the electrophotographic copying machine characterized by comprising the step of developing latent electrostatic images into visible toner images with a developer, the aforementioned latent electrostatic images are first formed on the surface of a photoconductor comprising a dielectric material, the latent electrostatic images are developed to toner images with a developer, and then the toner images are transferred to an image-receiving member such as a sheet of paper and thereafter fixed thereto by the application thereto of heat and/or pressure. Thus, visible toner images can be formed on the image-receiving member.
In recent years, with the increase of demand for the formation of full-color copy image, the consumption of a full-color toner is increasing. To obtain a full-color image, a document is subjected to color separation using a scanner, and the color image data read by the scanner is recorded using a laser beam, and then light exposure is carried out. Each color image is developed with a yellow toner, a magenta toner, or a cyan toner, and further, optionally with a black toner. The thus obtained color images are superimposed to produce a full-color image.
The requirements for the color toner, which are not similar to those for the black toner comprising carbon black as a coloring agent, include color tone, tinting strength, light resistance, heat resistance, and transparency of a pigment or dye to be employed as the coloring agent in the color toner. In particular, the transparency of color toner is of great significance in order to obtain high quality clear color image because a color image is formed by superimposing a plurality of color toners, as previously mentioned.
In light of all the factors of light resistance, heat resistance and transparency, conventional yellow toners are considered to be still unsatisfactory. Therefore, color reproduction performance of conventional full-color images produced by the color copying machine is still poor, and in addition, when a full-color image is transferred to a transparent sheet such as an OHP film and fixed thereon, the projected image tends to be dull and poor in terms of saturation.
The benzidine pigment as disclosed in Japanese Laid-Open Patent Application 51-144627 is satisfactory with respect to the tinting strength, color tone, and transparency thereof. However, there is the problem from the viewpoint of environmental safety that such benzidine pigment is in danger of generating a carcinogenic material when heated.
For obtaining satisfactory color tone, tinting strength, light resistance, heat resistance and transparency of the yellow toner, it is proposed to employ a specific coloring agent, or the combination of coloring agents. For instance, a yellow toner comprising an allylamide based pigment is proposed in Japanese Laid-Open Patent Application 61-151550, which yellow toner is capable of stably producing high quality images free from fogging with excellent fixing properties during the continuous copying operation. Further, in Japanese Laid-Open Patent Application 3-55563, there is proposed a yellow toner comprising a quinophthalone pigment, which is excellent in spectral reflecting characteristics and charging characteristics, and therefore capable of always stably producing images.
However, none of the above-mentioned conventional yellow toners can completely meet all the requirements as shown below:
(1) assuming a clear yellow color with such sufficient spectral reflecting characteristics as to obtain excellent color reproduction performance. PA1 (2) showing sufficient tinting strength and transparency owing to high dispersibility of a coloring agent in a binder resin. PA1 (3) showing good light resistance. PA1 (4) showing such a sufficient heat resistance as to cope with the fixing step under the application of heat, thereby preventing the surface of an image-fixing roller from being dyed with the toner. PA1 (5) showing stable triboelectric charging characteristics regardless of change in ambient conditions. PA1 (6) capable of preventing a carrier from being stained with the toner so as not to cause a so-called spent phenomenon when the toner is used as a two-component developer.
Furthermore, there remain many problems with respect to the transparency required for a yellow toner for use in color electrophotography.